--- Daniele Gratteri <daniele_gratteri@inwind.it> wrote:
> Maybe it is a stupid question: a 'pull up' resistor is a normal resistor
> simply connected between Vcc and the signal line or I have to ask for a
> specific type of resistor?
It's not a stupid question so much as an english-language question. In
this case "pull-up" refers to the job that it is doing, not what physical
kind of component it is. A "normally open" or "normally closed" switch
refers to what kind of switch it is.
You do, I think, understand what a pull-up resistor is: a resistor (typically
in the range of a few K ohms) connected between a signal and Vcc. It holds
the signal clearly at a logical "high" signal, which, for TTL is, ISTR between
2.8 and 5.0VDC. Common values include 2.2K Ohms and 4.7K Ohms. I have also
seen 1.2K Ohms. Among other things, the resistor prevents a direct path
between Vcc and Ground (Earth) preventing circuit damage in typical cases.
You normally want them because some chips do not like to have their inputs
floating. They want to be high or low, but not hanging in mid-air.
-ethan
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